Motorized bicycles are known in the art. See, for example, U.S. Pat. No. 4,397,369, to Read; U.S. Pat. No. 5,361,863, to Goodwin; U.S. Pat. No. 7,284,463, to Rizzetto; U.S. Pat. No. 7,770,682, to Spanski; U.S. Pat. No. 8,051,937, to Pesenti; and Published Pat. Appl. US 2005/0039963, to Forderhase. A significant drawback to these designs is that they are heavy and overly complicated. Because of their weight, they are not practically operable by human power alone. Thus, they are not bicycles in the truest sense of being operable over any appreciably long distance via the leg-power of the rider. They are more akin to motorcycles and designed to be powered exclusively (or very nearly so) by the affixed engine. Because the transmission assembly that links the engine to the gear-train that drives the wheels is complicated, the prior designs are difficult to manufacture and cannot be easily implemented into existing assembly lines for making conventional bicycles or other human-powered vehicles (tricycles, 4-wheelers, etc.). Thus, there remains a long-felt and unmet need for a lightweight transmission of a design that enables a vehicle to be powered with equal facility either by the human rider or by an engine.